THE role of microfilaments in secretion has been much debated^1–4. If secretion involves a sliding filament mechanism similar to muscle contraction⁵, microfilaments must attach at least transiently to vesicle membranes. Indeed, there is evidence that contractile proteins are associated with secretory vesicles (chromaffin granules) of adrenal medulla^6,7. Moreover, the outer (cytoplasmic) surface of secretory vesicles must provide ‘anchoring molecules’ for microfilaments. In skeletal muscle, the structural protein α-actinin located in the Z-line⁸ serves as such an anchoring molecule. An α-actinin-like protein was isolated from a variety of non-muscle cells^9,10 and there is evidence that the same protein is attached to the inner (cytoplasmic) surface of the plasma membrane of non-muscle cells in association with microfilaments^11–14. Using a monospecific antibody to porcine α-actinin (Fig. 1) we have demonstrated the presence of this protein in secretory vesicle membranes from chromaffin cells of the adrenal medulla and from platelets.